Microphone apparatus



B. BAUMQZWEIGER MICROPHONE APPARATUS Filed Dec. 20, 1937 I an mzwePatented Dec. 19,1939

PATENT OFFICE"! mcnornona srmmrrvs Benjamin Baumzweiger, Chicago, 111.,assignor to S. N. Shure and'Frances Share, trustee, doing business asSlim-e Brothers, a partnership Application December 20, 1937, Serial No.180,895

' Claims (Cl. 179-1) This invention relates to microphone apparatus andmore particularly to a microphone employing stifiness-controlledtransducers and having uni-directional characteristics,

In many acoustical applications it is useful to have a microphone whichis sensitive to sound Waves incident upon it from one direction, andwhich is notably less sensitive to sound waves incident upon it fromother directions. Such a result may be obtained by providing a pair ofelectro-acoustic transducers, one ofwhich is actuated by the differencein pressure between two points along the path of a sound wave, and theother of which is actuated by the pressure at' a point in the path ofthe sound wave. Fo venience, the first type of transducer referred to,will be called the pressure-difference operated electro-acousti'ctransducer, and the last mentioned type will be called the pressureoperated electro-acoustic transducer. I

An object of this invention is to provide an improved microphone,whichemploys a combination of transducers having different directionalcharacteristics to obtain an overall uni-directional result. A specificobject is to make the combination of a stiffness controlledpressure-difierence operated transducer anda stifiness-controlledpressure-operated transducer available in a microphone suitable for usewith standard amplifying equipment. Any of the well-knownstiiimess-controlled transducer movements can be used in conjunctionwith my invention, condenser, carbon, or piezo-electric crystal typesbeing cited as examples.

Another object is to provide simple means by which such a pair oftransducers may be connected in proper relation so that their outputcurrents will be in proper correspondence to pro vide the desiredoverall directional characteristics.

tional microphone comprising a pair of transducers of difierentdirectional "characteristics, in

which the correspondence of output currents is Still another object isto provide a uni-direc-' given pressure is quite independent of theambient temperature throughout the usual range of room temperatures. Theinternal impedance of the crystal itself, however, changes in a ratio ofthe order of 3 to 1 throughout the same temperature range. One of theimportant objects of my invention is to provide a'network in conjunctionwith Rochelle salt crystal transducers of different directivities toachieve correspondence of voltages for proper uni-directional action atall important sound-frequencies, and throughout the usual range 0! roomtemperatures.

Since a uni-directional microphone is composed in general of twotransducers, one of which presents essentially non-directionalcharacteristics at all important frequencies, and the other' one ofwhich presents essentially bi-directional characteristics at allimportant frequencies, it is often advantageous to be able' to use oneof the transducers at a time to permit a choice of directionalcharacteristics. Still another object of my invention is the use oftransducers of impedance such that a suitable switching device can beinserted into the' electrical circuit including the related electricalnetwork without introducing difliculties inherent to switching at veryhigh or very low impedances. arrange amulti-directional transducer, inwhich the directivity can be controlled by a simple switching devicehoused inthe microphone case.

Other objects of my invention will become apparent as the specificationproceeds.

An embodiment of the invention is illustrated in the accompanyingdrawing, in which Figure 1 is a-schematic view showing the transducersin cross section and indicating the electric circuit; Fig. 2, adiagrammatical view showing the equivalent electric circuit; Fig. 3, aview illustrating the directional characteristics of thepressure-difference operated transducer, the view being taken withreference to a substantially horizontal plane; Fig. 4, a viewillustrating the directional characteristic of the combinedpressure-diiierence operated transducer and pressure-operated transducerwhen the apparatus is adjusted, for uni-directional characteristics asdescribed elsewhere in this specification; and. Fig. 5, a circuitshowing a modified embodiment of the invention.

As shown in Figure 1, A designates a pressurediiference operatedtransducer; B, a pressureoperated transducer; and N, an electricalnetwork which corrects the frequency response of the transducer A so asto be suitable for combination with the output of the transducer BAnother object is to in order to obtain the desired directionalcharacteristics.

The pressure-difference operated transducer A is disposed in uprightposition and has a pair of diaphragms l0 and Ill facing front and backdirections respectively. Each of these diaphragms is mechanicallyconnected to the piezoelectric crystal ll so as to actuate the crystalupon movement of the diaphragms by the sound waves. A pair of conductorsl2 and I2 are connected with crystal II and receive the voltagegenerated by the transducer. It is apparent that the voltage generatedby transducer A, hereinafter known as E1, is proportional to thedifference in the sound pressures on each of the two diaphragms. Beingproportional to the pressure-difference between two points in space,this voltage is seriously affected by a change in the frequency of thesound wave, though the pressure of the sound wave remains constant.

The pressure-operated transducer B is shown immediately below thetransducer A with the diaphragm l3 in a position substantiallyhorizontal. However, the position of thepressure-operated transducer Brelative to the pressure-gradient element A may be altered if sodesired, and displacement from the above described position -will servethe purpose of bringing together or apart the acoustic centers ofsymmetry of the transducers, and thus alter the combined directionalcharacteristics of the two. Such displacement is to be regarded aswithin the limits of this invention.

The diaphragm I3 is connected mechanically with the piezo-electriccrystal I4, and conductors l5 and I5, connected with the crystal, areadapted to receive the voltage E2 generated by'this transducer. Thecomplete microphone assembly,

including the transducers, the electrical network elements andthedesired switching device, is.

preferably included within a suitable housing 22 shown in dotted line.It is evident that the electrical network elements and switching devicemay be placed within a separate envelope if desired.

Each of transducers A and B have capacitive internal impedance, and inthe equivalent electrical circuit shown in Fig. 2 the transducer A isrepresented by the alternator l6 which generates voltage E1 togetherwith its equivalent internal capacitance C1; and the. transducer B isrepresented by the alternator H, which generates voltage E2, togetherwith its internal capacitance C2.

The cable D by which the electrical impulses are delivered to thereceiving equipment may be of any suitable construction. For reasonswhich appear more clearly hereinafter, this cable should preferably havea predominant shunt capacitance of between 100 to 2000micro-microfarads. If desired, this conducting line or cable maycomprise a wire l8 which is insulated from and encased in a sheath I9which is grounded and forms the other side of the line. The shuntcapacitance of this conducting line is represented in the equivalentcircuit of Fig. 2 by the capacitance C3. In the absence of sufficientparallel capacity of the conducting line, a condenser of suitable valuemay be inserted.

In connecting the cable with the transducers, I prefer to use athree-way switch 20 which may be of any suitable type. The threepositions of this switch are designated in Fig. 1 as l, 2 and 3. It maybe stated here that for uni-directional operation the switch is inposition I, and Figs.

2 and 4 are indicative of circuit conditionsand directivitycharacteristic, respectively, when the switch is in this position.

With switch 20 in position I, the pressure-difference transducer A isconnected to the conducting line D through a parallel combination of aresistor R which may have an impedance value between 0.5 and 10 megohmsand a condenser C with a capacitance value between 10 and 100micro-micro farads, and which with the capacitance C3 comprises theelectrical network N by which the voltage developed at transducer A ismade to represent the intensity of the sound energy received, accordingto principles set forth in my copending application Serial No. 172,840,on Microphone apparatus, filed November 4, 1937. Preferably, theimpedance of- R should be equivalent to the total series capacitance ofthe circuit including transducer A at a frequency in the lower part ofthe audio-frequency range, such as, say, 100 cycles per second, and thecapacitive reactance of C made equal numerically to the resistance of Rat a frequency subsequently referred to as the critical frequency, atwhich half the wave length of sound equals the effective acousticalfront-to-back distance of transducer A. This distance may be taken asthe shortest front-to-back path around the supporting frame, from thecenter of one diaphragm to the other, or, if one of the diaphragms wereeliminated, from the center of 'the'single remaining diaphragm on oneside,

around the frame to the center of said diaphragm on the other side. Inthe transducer shown as A this front-to-back distance, subsequentlycalled 11, is approximately three inches, thus providing a criticalfrequency of 2200 cycles per second. As explained in my copendingapplication previously referred to, at frequencies above the criticalfrequency the transducer A operates substantially as a simple pressuredevice because of the obstacle effect" which obtains because of itssize, and the distance d can be made to be any amount depending uponavailable space, and other considerations therein discussed, say, forexample, between one and five inches.

Also, at this position of switch 20, the pressure-operated transducer Bis connected to conducting line D' through a condenser C4 which ispreferably of the adjustable type and which may have a value of between40 and 500 micro-micro farads. With this arrangement, it is seen thattwo parallel branches are formed in the circuit, one branch includingthe pressure-difference operated transducer A and the parallelcombination of resistance R. and condenser C, and the other branchincluding the pressure-operated transducer B and the condenser C4, andthat each of these branches is connected to the conducting line havingthe shunt capacitance Ca.

If switch 20 is turned to position 2, it is seen that thepressure-operated transducer will be disconnected from the circuit andonly the pressure-difference transducer will be connected to theconducting line. If switch 20 is turned to position 3, thepressure-difference transducer will be disconnected from the circuit andthe pressure-operated transducer B alone connected to the line, thevariable condenser C4 being also excluded from the circuit. It should benoted that it is not necessary to exclude condenser C4 in order to usethe pressure-operated transducer alone, but it is advantageous to do soin order to obtain the maximum voltage at the receiving apparatus.

'aiaearr sensitive to sound waves approaching from bothfront and rear.It will be observed that the directional sensitivity varies as thecosine of the angle of incidence, and the directivity characteristicsmay be indicated as shown in Fig. 3. Due to the. presence of thecomparatively large resistance R, the effective voltage across the shuntcapacitance C3 is rendered substantially independent of sound frequencyat most frequencies in the audio range. A more elaboratedemonstration ofthis, fact will be found in my copending application Serial No. 172,840for Microphone apparatus, filed November 4, 1937.

When switch 20 is in position I, the outputs of the two transducers willbe combined and the effective voltage Er will be-the vector sum of thevoltages due-to each 'of the transducers. The purpose of the adjustablecondenser C4 is to enable the outputs of the two transducers to bebalanced so as to secure the desired directional characteristics.

By applying well known electrical laws, it may be shown that theeffective voltage Er, when switch 20 is on point I, may be given verynearly by the equation:

where P is the root-mean-square pressure of the sound wave,

. a is the effective area of one of the diaphragms .E2 is theroot-mean-square voltage developed in the pressure-operated element,

I R is the value of the series resistance,

Cp is the equivalent series capacity of the pressure element,constituted by a series combination of C2 and 04, I

C3 is the shunt capacitance of the conducting line, and

0 is the angle of incidence of the sound wave upon the pressure gradientelement.

When the sound waves approach the microphone directly from the front, 0is zero, and cos 0 is unity, hence. the effects of the potentials due tothe pressure and pressure-difference units are additive. If 6 is 180,then cos 0 is -1, and the effects of the two potentials are subtractive.I prefer to make the value of Cp such that the two terms of thenumerator of Equation I will be equal and oppositein sign when 0 is 180(corresponding with a sound wave approach toward the rear diaphragm I0).

Thus, the overall effective voltage developed or the combined responsewill be substantially zero to sound'waves approaching directly from therear of the microphone. For this condition the directionalcharacteristics of the microphone will be represented by the curvedescribed by the 3 equation p=(1+cos 0), which, as shown in Fig. 4, isthe well known cardioid curve.

Referring again to the Equation I it is seen that C is a seriescombination consisting of relatively large crystal capacity C: and arelatively small capacity C4. Hence a change in C2, as might be producedby temperature change, would have practically no eflect upon themagnitude of Cp, and such a change would only slightly affect thedirectivity characteristics.

It is understood that means other than condenser C4 may be employedforobtaining correspondence between the outputs of the two trans-.

ducers for overall uni-directional characteristics.

For exampe, as is seen by Equation I, the value of R. may be adjustedfor such operation. However, since the frequency response characteristicof transducer A, at least throughout a part of the audio spectrum,depends on the value of R, and also because of mechanicalconsiderations, I prefer to adjust the system by a change of the seriescapacitance in the pressure-operated transducer branch of the circuit.By using a variable condenser in this branch of the circuit. the systemmay easily be adjusted for the desired directional characteristicswithout altering the a desired action of the electrical network N.

It is apparent that-by adjusting the value of the added conden er C4,the relative importance of the pressure-operated element may be changed,and the overall directional characteristics accordingly altered.

By operating switch 20, the directivity characteristics of the"microphone may be made either non-directional, by-directional, oruni-directional, as may be desired for any particular use orapplication.

The above description is not intended to restrict the invention to thespecific circuits shown diagrammatically in Fig. 1, and its electricalequivalent shown in Fig. 2, since it is evident that the outputs of thetransducers can also be combined in what may be termed a seriesarrangement. Thus Fig. 5 shows an alternative equivalent circuit inwhich the voltage developed across condenser Ca due to bi-directionaltransducer I6 is added to the voltage drop across condenser C3 due tonon-directional transducer. I I by a series arrangement of thecondensers. Condenser C4 is as before used to adjust the voltagedeveloped by the non-directional transducer across its condenser C3". Inthis particular modification, I have found that the voltage adjustingcondenser C4, along with C3", may be dispensed. with entirely, leavinggenerator I! in series with the effective output of transducer i6 asdeveloped across condenser C3, and the output leads of the microphone,provided generator Il is designed to produce the exact voltage requiredfor the desired directional characteristics. However, in this particularmodification, I prefer to use the arrangement shown in equivalent inFig. 5 because of practical considerations.

I have described only two embodiments of the invention and it isapparent that many changes may be made in the apparatus as describedwithout departing from the spirit of the invention. The values of thecircuit constants given in this explanation are not to be regarded asexclusive or as constituting limits of the invention.

The foregoing detailed description has been given, for clearness ofunderstanding only and no unnecessary limitations should be understoodtherefrom, but the appended claims should be construed as broadly aspermissible-in view of the prior art.

I claim: I

1. In a microphone, a pressure-difference operated piezo-electrictransducer; \an electrical circuit including the pressure-differenceoperated piezo-electric transducer and microphone output conductorspresenting a shunt capacitance of 200m 2,000 micro-micro farads, and aresistor having a resistance value between 0.5, and 10 megohms connectedin series with and between the above-mentioned transducer and themicrophone output conductors; a pressure-operated piezo-electrictransducer connected in series relationship with a variably adjustablecondenser' of 20 to 200 micro-micro farads capacitance, which seriesarrangement of -transducer and condenser is connected across themicrophone output conductors,

2. In a microphone, a piezo-electric transducer operating by differenceof pressure at two regions in a sound wave, the effective acousticaldislance between said regions being substantially greater than one-halfthe wave length of the highest frequency to be received; a circuit be:tween the above-mentioned transducer and the receiving apparatus, whichcircuit includes a conducting cable presenting shunt capacitance, and aparallel combination of a resistor and a condenser connected in seriesbetween the abovementioned transducer and the conducting cable, theresistor having a resistance value numerically equal to the capacitivereactance of the cable at a relatively low frequency in the audio range,and the condenser having a reactance equal to the res'stance value ofthe resistor at a frequency for which the effective acoustical distancebetween the two regions in the sound wave is substantially equal toone-half the wave length; a piezo-electric transducer operated by thepressure of the sound wave at a region in the vicinity of theabove-mentioned two regions; an electrical circuit for interconnectingthe transducers and the conducting cable; and adjusting means forproviding equal output voltages from both transducers at; the conductingcable for sound incident from a given direction.

3. In apparatus of the character set forth in claim 2, adjusting meansconsisting of a variably adjustable condenser in series relationshipwith one of the transducers, for providing equal output voltage 1 fromthe transducers at the conducting cable.

4. Apparatus of the character set forth in claim 2, in which the valuesof the circuit elements are as follows: equivalent parallel capacitanceof the conducting cable, 200 to 2,000 micro-micro farads; resistancevalue of the resistor in the parallel branch, 05 to 5 megohms;capacitance of the condenser in parallel with the above-mentionedresistor, 20 .to 200 micro-micro farads; distance between thepressure-sensitive regions of the pressure-difference transducer, 1 to 5inches.

5. In a microphone, a pressure operated transducer having substantiallyuniform frequency response, a transducer operating by difference ofpressure at two regions in a sound wave, the effective acousticaldistance between said regions being substantally greater than one-halfthe wave length of the highest frequency to be received, saidsecond-mentioned transducer having a rising frequency response up to acritical frequency within the audio range and a substantially constantfrequency response above said critical frequency in circuitcombinationwith a network inserting a loss proportional to frequencybelow the critical frequency and independent of frequency above thecritical, and circuit means for interconnecting the transducers so thatthe voltage output at the terminals of the device for sound incidentfrom a given direction is substantially less than for sound wavesapproaching from the opposite direction, throughout a substantial rangeof audio frequencies.

6. In a piezo-electric transducer operate'd'by' difference of pressureat two regions in a sound wave; an electrical circuit including thepressuredifference operated piezo-electric transducer andmicrophone'output conductors presenting a shunt capacitance of more than200 micro-micro farads, and a resistor having a resistance value between0.5 and 10 megohms connected in series with and between theabove-mentioned transducer and the microphone output conductors apressure-operated piezo-electric transducer connected in seriesrelationship with a condenser of 20 to 200 micro-micro faradscapacitance, which series arrangement of transducer and condenser isconnected across the microphone output conductors.

7. Apparatus of the character set forth in claim 2, in which the valuesof the circuit elements are as follows: equivalent parallel capacitanceof the conducting cable, more than 200 micro-micro farads; resistancevalue of the resistor in the parallel branch, 0.5 to 5 megohms;capacitance of the condenser in parallel with the abovementionedresistor, 20 to 200 micro-micro fax-ads; distance between thepresure-sensitive regions of the pressure-difference transducer, 1 to 5inches.

8. In apparatus substantially as described in claim 2, there being aneffective acoustical distance between said regions of 1 to 5 inches.

9. In apparatus substantially as described in claim 1, switching meansadapted to combine the transducers so that either the unidirectional,bi-

directional on nondirectional characteristic can be selected.

.10. In a microphone, a transducer operating by difference of pressureat two regions in a sound wave, the effective acoustical distancebetween said regions being substantially greater than onehalf the wavelength of the highest frequency to be received; a circuit between theabove-mentioned transducer and the receiving apparatus, which circuitincludes a conducting cable presenting shunt capacitance, and a parallelcombination of a resistor and a condenser connected in series betweenthe above mentioned transducer and the conducting cable, the resistorhaving a resistance value numerically equal to the capacitive reactanceof the cable at a relatively low frequency in the audio range, and thecondenser having a reactance equal to the resistance value of theresistor at a frequency for which the effective acoustical distancebetween the two regions in the sound wave is substantially, equal toone-half wave length; a transducer operated by the pressure of the soundwave at a region in the vicinity of the above-mentioned two regions; andan electrical circuit for inter- BENJAMIN BAUMZWEIGER.

CERTIFICATE OF CORRECTION. Patent'No. 2,, l 8h,21 7. December 19,1939'.-

BENJAHIN BAUHZWEIGER.

It is hereby certified that error. appears in the printed specificationof.the above numbered patent requiring correction as follows: Page 1;,sec-- 0nd column, line 12, claim 6, after "mo" 'ina'ert microphone, a;and that the said Letters Patent should be read with thiscorreetionthe'rein that the seine may oonformito' the record of the caseiri the Patent Office.

Signed and sealed this 0th day or January, A. D. 19110.

Henry Van Aredale (Seal) Acting Commissioner of Patents

